Starting circuit for electric discharge lamps



March 15, 1960 e. E. v. SiLJEHOLM ETAL 2,928,990

STARTING CIRCUIT FOR ELECTRIC DISCHARGE LAMPS File d Dec. 19, 1956 3Sheets-Sheet 1 STARTING PERIOD sr-zc z 5 March 15, 1960 Filed Dec; 19,1956 G. E. V. SILJEHOLM El" AL STARTING CIRCUIT FOR ELECTRIC DISCHARGELAMPS 3 Sheets-Sheet 2 Fig.3

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G. E. v. SILJEHOLM EIAL 2,928,990

March 15, 1960 STARTING CIRCUIT FOR ELECTRIC DISCHARGE LAMPS 3Sheets-Sheet 3 Filed Dec. 19, 1956 WU figb 255 1 STARTING PERIOD 5P SEC1,0

33 34 ,32 H Tf STARTING CIRCUIT FOR ELECTRIC DISCHARGE LAMPS cam is. v.Siljeholm, Bromma, and like KAG. Bjiirkman, Alvsjo, Sweden, assignors toLumalampan Aktiebolag, Stockholm, Sweden, a corporation of SwedenApplication December 19, 1956, Serial No. 629,337 Claims priority,application Sweden December 21, 1955 v 6 Claims. (Cl. 315-07) Thisinvention refers to a starting circuit for electric discharge lampswithout using magnetic relays, thermal or glow switches.

In electric fluorescent lamps especially it is-desirable to provide foran instant start, like that provided in case of electric incandescentlamps. A known device for instant start of fluorescent lamps includespreheatable glow electrodes fed by an inductance-capacitance circuit atresonant frequency, the heating of the glow electrodes being efiectedeither by the resonance current passing through the glow electrodesconnected in series or by an inductance coil, called oscillatory choke,provided with glow current coils for heating the glow electrodes. As arule, up to now the oscillatory choke was made as a leakage transformerto prevent the glow current coils from interfering with the resonancevoltage.

In circuits of the type above referred to, the change in the resonanceposition is small during the preheating period, and the voltage dropacross the discharge lamp produced by resonance is practically constantand independent of the temperature, to which the .glow electrodes areheated. To ensure the starting of the discharge lamp, this resonancevoltage must be chosen relatively high, e.g. 300 volts for a 40 wattfluorescent lamp, which, however, is so high that it can causecold-start of the fluorescent lamp, i.e. start at a premature stage ofthe preheating period when the glow electrodes are insufficientlyheated. Such cold-start involves a very great stress on the glowelectrodes, so that the life of the fluorescent lamps are considerablyshortened. If, on the other hand, the resonance voltage is chosen so lowthat no cold-start is to be fearede.g. 265 volts in this case thedischarge lampwill not start under all conditions without failure. Dueto these risks of cold-start and failure to start, respectively, thecircuits mentioned above did not find any proper practical use.

This invention involves a radical departure from the principlesdescribed above, granting a sure start of, as well as a long life to thedischarge lamp. The new starting circuit is intended for electricdischarge lamps with glow electrodes that are preheatable, haveresistances depending on the glow temperature, and are fed by glowcurrent coils in a transformer section of a resonancetuned LC-circuitfor one or seveal discharge tubes. Characteristics of the invention arethat the glow current coils are coupled to the primary coil of thetransformer, so that the change in the resistance of the preheatableglow electrodes can be utilized for transforming the resonance voltageapplied to the discharge lamp or lamps, from a low value precludingcold-start during the preheating of the glow electrodes, to a valueensuring the start of the discharge lamp or lamps in the finalpreheating stage.

In order to attain this result there must be a tight coupling betweenthe glow current coils and the primary coil of the transformer. Thismakes it possible to design the circuit so that the voltage across thedischarge lamp has a low value, and that, by heating the glowelectrodes, this voltage rises to a permissibly higher value, ensuring areliable start under all external conditions. This result is attained byutilizing the change in, resistance of the glow electrodes caused bystarting from an initial cold condition up to a final hot condition. Inthis process an approximately tenfold increase of resistance may takeplace with glow electrodes of the common type. By virtue of the couplingof the glow current coils and the oscillatory choke this change inresistance" affects the electric current in the oscillatory choke sothat its impedance rises from a low value for cold electrodes to a highvalue for hot electrodes This hypothesisled to the invention. Laterexperience may possibly lead to a modified theory of the basicphenomenomthe steerage of the starting voltage by means of the glowelectrodes. -i The invention is'clarified by the following descriptionof a few examples shown in the attached diagrams. Fig. 1 shows a wiringdiagram for operation of a discharge lamp according to the invention.Fig. 2 shows a sche matic diagram for certain data of this wiringdiagram compared with data for the old wiring diagram described above.Fig. 3 shows a wiring diagram for operation of two discharge lampsaccording to the invention. Fig. 4 shows a schematic diagram for data ofthe wiring diagram given by Fig. 3. Fig. 5 shows one more wiring diagramfor operation of two discharge lamps according to the invention.

In the wiring diagram according to Fig. 1 an A.C.-fed discharge lamp 1,a capacitor 2, and impedances 3 and 4 are matched to each other and tothe frequency of the main voltage in such a way that resonance voltageis produced across the lamp and lights it. The choke 3 is in series withthe lamp, while a line containing an oscillatory choke 4 and capacitor2, shunts it. The choke 3, the capacitor 2 and the oscillatory choke 4constitute a con-1 trol circuit for the discharge lamp 1. Theoscillatory choke 4 is constructed as a transformer with two glowcurrent coils, 5 and 6, for feeding the preheatable glow electrodes, 7and 8, of the lamp 1.

By coupling the primary and the secondary coils of the transformer 4 astight as possible and by inserting a capacitor of the proper size inseries with the primary of the transformer, it is possible to choose astarting voltage suitable for the discharge lamp independent of thevalue of the main voltage, and to phase-compensate the circuit so thatthe currents through the discharge lamp and through thechoke are equal.I

The wiring diagram of Fig. 1 illustrates a starting circuit for theoperation of a 40 watt fluorescent lamp. 1 .On the basisof hisexperience an expert can draw the necessary conclusions from thisexample for applying the invention to other cases.

When a voltage of 220 volts is applied to the connection terminals, acurrent will pass through the choke 3, the capacitor 2, and thetransformer windings 4, 5 and 6 with a magnitude determined by thegeometric sum of the resistances of the respective components. 'If thechoke is standard for a 40 watt lamp, the condenser and the transformerare designed for resonance, so that at most 200 ma. may pass through thecircuit and so that the voltage drop across both of them'becoines 260volts, i.e. the voltage drop across the tube will be 260 volts at theswitching moment. The glow current C ils of the transformer are designedso that at that moment a glow voltage of 10 volts is set up across everyglow electrode. In a case chosen as an example the transformer coilsconsist of a primary coil of 2000 turns of 0.22 mm. wire and two turnsecondary coils of 0.43 mm. wire wound on a 20 watt mandrel. Whenthelamp lights, a lamp current passes through the choke increas" ing itsinductive reactance considerably, shifting consider;

ably the resonance position of the circuit and reducing the currentthrough the primary coil of the transformer to about 30 ma. Thiscurrent, which is capacitive, sets up a constant glow current of about 3volts in the respective glow electrodes and has a phase-compensatingeffect upon the choke, so that the network current is maintained at thesame value as that of the lamp current, i.e. at about 0.42 A. Thecondenser of 1.0-1.2 ltf. capacity has in the beginning a voltage of 330volts and of approximately 80 volts in operation. The voltage across thelamp, when the cold resistance of the glow electrodes is 2 times 2.5ohms, is 260 volts, as indicated above. While the electrodes are glowingthey have a total resistance of 2 times 25 ohms, and the voltage acrossthe lamp is approximately 300 volts.

'Qrily y m king the tran f rm r with a tigh c up ing it ifs possible toutilize the change in resistance of the glow electrodes y ting t em f omro m o slo temperature. This change may be considerable, eg. 2.5:25ohms. Due to this change in electrode resistance an increase in load iscreated in the secondary circuit of the transformer, which during thecritical period between the switching on of the circuit and the lightingof the lamp with its full intensity reacts on the primary of thetransformer.

tion, as shown in Fig. 2, to obtain by changes of impedance of thetransformer a voltage drop across the fluorescent lamp of onlyapproximately 260 volts in the moment of connection, which successivelyincreases to approximately 300 volts with the temperature of the glowelectrodes rising from room temperature to approximately 800 C. In Fig.2 the time for lighting in seconds (r is set off as abscissa, thecathode temperature in C., and the voltage across the lamp in volts(curves 9 and 10) as ordinates. In practice, by this invention avaluable technical effect is obtained, implying not only that cold-startis precluded, but also that the high impedance of the transformerprecludes every influence of the condenser on the form of the curve ofthe lamp current. By measuring the form factor of the described schemeit was found to be approximately the same as that of an ordinary chokeconnection, e.g. 1.46.

According to the diagram in Fig. 2 initiation of the discharge occurs atpoint 11, corresponding to a glow electrode temperature of 700 C. orhigher. The voltage across the lamp that a leakage transformer wouldsupply is constant during the whole preheating interval, as indicated bythe dash-dot line 12. Premature initiation of the discharge occurs atpoint 13 on the temperature curve, corresponding to 500 C., which is toolow; but still more unfavourable cold-starts can occur due to the factthat such a high voltage has to be applied to the lamp.

The invention is, as indicated above, not limited to starting circuitsfor operation of single lamps but can also be used to operate severaldischarge lamps. 3 shows an example of such use for initiation andoperation of two lamps 14 and 15 by a LLC-circuit. In this case theoscillatory impedance 16 has three glow current coils 17, 18 and 19, allcoupled with theprimary coil 20 in such a way that the change in itsself-inductance occurs according to desire, at the start causing thevoltage across the discharge lamps to rise from a low value that doesnot cause a cold-start, to a high value that releases the ignitionwithout fail. The specific characteristics .of the invention show upadvantageously, particularly in such a circuit with several glow currentcoils. It ,is important to bring about the change .of impedance desiredin the primary coil 20, so that the voltage change mentioned'takes placeacross the discharge lamps. Herein itisnecessary to consider'carefull-yall dimensions and 186019 it-that thec'ouplings between the differentcoils are adjusted inrsuch away that noundesirable changeinthe;sclf-inductance.of.thelprimarycoilmayiariset By suitable choice ofthe sizes of the components contained, it is possible according to theinven- 4 can easily arise, if the change of the inductance is so greatthat the resonance position of the circuit is displaced too much. InFig. 4 curve 21 shows such an undesirable case for voltage change acrossone, respectively several lamps, whereas curve 22 shows the result aimedat with a correct design.

Operating two 40 watt fluorescent lamps in parallel according to thewiring diagram shown in Fig. 3, the components had on one occasion thefollowing data: the L-choke 2'3 consisted of 1700 turns of 0.43 mm.wire, the air gap of the mandrel being 2.0 mm. The LC-choke 24 had 1320turns of 0.43 mm. wire and an air gap of 1 mm, the C-condenser 25connected to it in series having a capacity of 3.7 f. The condenser 26of the shunting line had a capacity of 2.0 f, and the primary coil ofthe oscillatory impedance 16 con sisted of 1850 turns of 0.25 mm. wire.Each secondary with; a d 1.9 te t ng one le t-wiles? the C and of heblame. respecti l s n s sd 9? 1 -9 tu n o 0-3 mm. wire, while thesecondary coil 17, feeding the two remaining electrodes in parallelconnection, consisted of lIOturns of 0.40 mm. wire. The condenser 27between the L and the C-lamp had a capacity of 0.5 f.

The wiring diagram'in Fig. 5 refers to connection in series. Using it tooperate two 20 watt fluorescent lamps on 220 volts, the following datamay serve as an example: the choke 28 had 1550 turns of 0.40 mm. wireand l rnrn air gap. The condenser 29 in the line shunting the two lampshad a capacity of l.8/;rf., and the primary coil 31 of the oscillatoryimpedance in series with it had l 0 turns of 0.25 mm. wire on a 20 wattmandrel. Each of the two coils 32 and 33 out of the three secondarycoils 32, 33, and 34 of the transformer, feeds one electrode of therespective lamp, and the third coil 34 the two remaining electrodes inparallel connection. The coil 32 as well as 33 had turns of 0.35 mm.wire and coil 34 turns of 0.40 mm. wire. If, according to the idea ofthe invention, sufficient control of the voltage across the dischargelamp is .obtained merely through one or a few glo'w electrodes by way ofthe respective glow current coils, it is permissible to have theremaining glow current coils with a looser coupling or even to have aleakage transformer. Thus, for example in the circuit with two dischargelamps in series as described above with reference to Fig. 5, the coils32 and 33 of the transformer 30 can be so tightly coupled that a voltageacross the fluorescent lamp which changes according to curve 22 in Fig.4 is obtained. The ,coil 34 may then, if so desired, be coupled veryloosely to the primary coil 30. The ignition course desired according tothe invention is nevertheless obtained. In such an arrangement ofmultilamp connections one glow electrode in each discharge lamp shouldsuitably contribute to the steerage of the starting voltage from a lowto a high value in the way intended.

What we claim is:

1; A starting circuit for electric discharge lamps comprising a sourceof alternating current, a lamp control circuit connected across saidsource, said lamp control circuit including a choke coil, a capacitorand a primary coil, an electric discharge lamp supplied by said sourceand having a coupling connection with said primary coil, said electricdischarge lamp having preheatable electrodes the resistances of whichare positively variable according to temperature, saidcouplingconnection including a pair of glow coils, each of said glowcoils being connected across one of said 'preheatable electrodes and atleast one of said glow coils being tightly coupled to said primary coilwhereby any change of resistance in the preheatable electrode which isconnected across said closely coupled glow coil is transferred as changeof impedance'to said primary coil in the lamp control circuit, said'lampcontrol circuit being-normally capacitive and adapted tobe resonantwhereby any increase in the impedance pf said primary coilwill-cause thecurrent therein to increase, the resulting increase in the voltage dropacross said primary coil being applied to the 'j dis'- charge lamp bysaid coupling connection as said electrodes are heated.

2. A starting circuit for electric discharge lampsQ-as claimed in claim1, wherein there is provided a second electric discharge lamp having apair of preheatable electrodes the resistances of which are positivelyvariable according to temperature, said second electric discharge lampbeing disposed between the first mentioned lamp and the other side ofsaid alternating current source, one of the electrodes in said secondelectric discharge lamp being series connected with one of theelectrodes in said first mentioned electric discharge lamp. J

3. A starting circuit for electric discharge lamps as claimed in claim 2wherein there is provided a third glow coil, said third glow coil beingarranged in tight coupling with said primary coil, one of the electrodesin said second electric discharge lamp being connected across one of theglow coils for the first mentioned lamp and the other electrode in saidsecond discharge lamp being connected across the third glow coil. 2

4. A starting circuit for electric discharge lamps as claimed in claim 1wherein there is provided a second electric discharge lamp having a pairof preheatable electrodes the reistances of which are positivelyvariable according to temperature, a second choke coil connected to oneside of said alternating current source, a second capacitor and one ofthe electrodes in said second choke discharge lamp being'seriesconnected to said second choke coil, a third glow coil coupled to saidprimary coil, said one electrode in the second electric discharge lampbeing also connected across said third glow coil, and the otherelectrode .in said second electric discharge lamp being connected acrossone of the glow coils for the first mentioned lamp and to the other sideof said alternating current source.

5. A starting circuit as claimed in claim 4 wherein all of said glowcoils are arranged in tight coupling with said primary coil.

6. A starting circuit as claimed in claim 5 wherein a third capacitor isconnected between a pair of electrodes each of which are therefordisposed in one of said electric discharge lamps.

References Cited in the file of this patent UNITED STATES PATENTS

